Printk

printk is a C function from the Linux kernel interface that prints messages to the kernel log. It accepts a string parameter called the format string, which specifies a method for rendering an arbitrary number of varied data type parameter(s) into a string. The string is then printed to the kernel log.

It provides a  abstraction and its parsing of the format string and arguments behave similarly to. It acts as a debugging tool for kernel programmers who need this function for logging messages from the kernel.

The  function prototype is:

C standard library and its  function are unavailable in kernel mode, hence the need for.

Differences from
The function  is based on , but cannot always be used in the same way that   is used.

Log levels
allows a caller to specify the type and importance of the message being sent. This specifier is called the log level.

The log level specifies the type of message being sent to the kernel message log. The log level is specified by prepending (using C's string literal concatenation) a string describing the log level to the start of the message to be produced. For example, a message could be produced at the  using the following:

The string specifying the log level consists of the ASCII start of the header character followed by a digit describing the log level or the character 'c' to indicate the message is a continuation of the previous message. The following log levels, along with their interpretations, are given below.

When a log level is not specified, the default log level is, unless a different default has been set in the kernel itself, such as with the   boot argument.

Log levels are defined in. Which log levels are printed is configured using the sysctl file.

Pointer formats
The  format specifier (used for printing pointers in  ) is extended to add additional formatting modes, for example, requesting to print a   using   would print an IPv4/v6 address and port in a human-friendly format (e.g. "1.2.3.4:12345" or "[1:2:3:4:5:6:7:8]:12345").

No floating point support
While  supports output of floating point numbers,   does not, since the Linux kernel does not use floating point numbers within the kernel.

Description
The function tries to lock the semaphore controlling access to the system console. If it succeeds, the output is logged and the console drivers are called. If it is not possible to acquire the semaphore the output is placed into the log buffer, and the current holder of the console semaphore will notice the new output when they release the console semaphore and will send the buffered output to the console before releasing the semaphore.

One effect of this deferred printing is that code which calls  and then changes the log levels to be printed may break. This is because the log level to be printed is inspected when the actual printing occurs.

The function  can be called from anywhere in the kernel except during the very early stages of the kernel boot process; when the system console is not initialised. The alternative function  is implemented on some architectures and is used identically to   during the early stages of the boot process.